Electrostatic immersion objective comprising an improved intermediate electrode



ug 17, 1955 H. DUKER 3,201,588

ELECTROSTTIC IMMERSION OBJECTIVE COMPRISING AN IMPROVED INTERMEDIATE ELECTRODE Filed Nov. 16, 1962 2 Sheets-Sheet 1 i E-`S il \-E\ ,A l

y l Figi.l

Aug. 17, 1965 H. DUKER 3,201,588

ELECTROSTATIC IMMERSION OBJECTIVE COMPRISNG AN IMPROVED INTERMEDIATE ELECTRODE Filed Nov. 16, V1962 2 Sheets-Sheet 2 F ig. S

Fig-7 United States Patent O tll ELECTRSTATIC MERSIN UBJECTWE CGM- PRlSlNG AN IMPRVED INTERMEDE'ATE ELEC- TRDE Heinrich Either, Stuttgart-lt/lohringen, Germany, assigner to Trab, Tauber Yr Co. nG., Zurich, Switzerland naar Nev. is, rss2, ser. Ne. Zeegse Claims priority, application Switzerland, Nov. 22, 196i, 13,5% /6l S Claims. (Cl. 25d-495) The invention relates to an electrostatic immersion objective, as required for example in electron emission microscopes.

The state of the art and an embodiment of the invention will now be described with reference to the accompanying drawings, in which:

FGS. 1, 2 and 3 show known forms of electrostatic immersion objectives;

FlG. 4 shows an electrostatic immersion objective constructed in accordance with the invention;

FG. 5 shows the electrostatic immersion objective of EG. 4 connected to a source of operating potentials;

FIG. 6 shows an enlarged cross-section of an intermediate electrode according to one embodiment of the invention; and

FIG. 7 shows an enlarged cross-section of an intermediate electrode and cathode according to another embodiment of the invention.

ln a known electrostatic yimmersion objective shown in FIG. 1, the object consists of a cathode K, a control electrode S and an anode A. The cathode K is at a strongly negative potential with respect to the anode; the control electrode S is at a potential which lies in the region ofthe cathode potential. The electrons emitted from the cathode K under some influence (temperature, bombarding with particles or electromagnetic waves) are accelerated by the electrostatic iield between the anode A and cathode K and pass through the hole in the anode A into the image space. Because of the controlling action of the control electrode S, the arrangement as a whole acts on the electrons like a converging lens, so that in the image space below the anode A an image of tie surface of the cathode K is produced by the emitted electrons. The object of the image is therefore the surface of the cathode K.

The resolving power ot such an arrangement is given by the formula denotes the resolving power, e is the mean velocity of the emitted electrons, E is the eld intensity in front of the cathode K and k is a numerical factor depending on the geometry of the objective and the nature of the object.

The velocity distribution and hence the value cannot be iniiuenced by the form of objective. To obtain good resolution, therefore, the factors E and k have to be intluenced. The field intensity E in front of the cathode K should be as high as possible, while the factor k, which includes the image forming properties of the objective, should be made as small as possible. Good image forming properties mean small lens errors.

The immersion objective shown in FlG. 1 represents a compromise which does not create optimal conditions. The electrostatic iield between the anode A and cathode K is heavily screened by the cont-rol electrode S. The smaller the hole in the control electrodes, the smaller is the penetration of the electrostatic iield therethrough and .the smaller the field intensity in front of the cathode K. lf, however, the hole in the control electrodes is made JCC larger, the image forming properties of the objective deteriorate and the factor k increases.

An experiment has been carried out (A. Septier, These, Paris, 1954), to improve conditions by inserting a further electrode Z at anode potential between the control electrode S and cathode K. ri`his gives the arrangement shown in FG. 2: the anode A and the intermediate electrode Z are at the same potential, lrelative to which lthe cathode K and control electrode S are strongly negative biased. The purpose of the arrangement is to separate the two factors E and k and to iniuence each individually. ln fact, the effective iield in this arrangement is produced mainly vbetween the cathode K and the intermediate electrode Z, while the three electrodes Z, S and A together form an electrostatic Einzel lens.

ln such an arrangement the iield intensity in front of the cathode K can now be intluenced by altering the distance between the cathode K and the intermediate electrode Z without the properties of the Einzel lens being affected. Conversely, the properties or" the Einzel len-s can be altered without affecting the field intensity in front of the cathode. Thus it must be possible to improve the resolving power.

The experiments result was negative; the resolving power could not be improved. Moreover, it was found ythat no reasonable images could be obtained by means of this arrangement Abecause. of unusually severe image distortions.

The new investigation which led to the present invention were started on the assumption that the principle of the immersion lens with four electrodes as shown in FG. 2 was in itself suitable for improving the properties of an emission microscope. The intention was therefore to ascertain what additional conditions had to be fulfilled by an. immersion objective consisting of four electrodes, and how far the negative result of the earlier experiments was caused by non-observance of these conditions.

According to the invention which resulted from .these new investigations, there is contemplated an electrostatic immersion objective comprising a cathode, an intermedlate electrode, a control electrode and an anode. The intermediate and control electrodes and the anode each having a bore or opening. The thickness of the intermediate electrode which lies between the cathode and the control electrode is, at least in the vicinity of its bore in it, at most a fifth of the distance between cathode and intermediate electrode. And the diameter of the bore in the intermediate electrode is at most a fifth of the distance between cathode and intermediate electrode.

The possibility of incorporating an intermediate electrode Z in an immersion objective with three electrodes in such a way that it improves the properties of the objective therefore depends primarily on the form of this intermediate electrode itself. 0n the basis of the investigations the decisive factor for the image quality is the homogeneity of the electrostatic field along the electron beam between the cathode K and the entry in the Einzel ens. Evidently this tact was not taken into consideration in the earlier experiments and homogeneity of the electrostatic field was only demanded for the space nearest the cathode K. The thickness of the intermediate electrode Z has no substantial inuence on this 'limite/d space. Similarly, no such rigorous requirements have to be laid down for the bore diameter. The construction of the immersion lens used for the earlier experiments follows this conception. Such a construction is shown in FIG. 3. The dimensions are as follows: the distance a between cathode K and intermediate electrode Z is from 1 to 2 mm. (millimeter-s); the thickness z of the intermediate electrode Z measures 2 mm. (millimeters); the diameter Dz of the bore in the intermediate electrode Z is 1 mm.

vin the Einzel lens.

abonnes e; 'i (millimeter) homogeneity of' the electrostatic field can be expected in the immediate vicinity of the cathode K, but at a short distance from the cathode K the electrostatic field becomes severely non-homogeneous, particularly when a=1/2z. The electron beam does not enter the electrostatic field of the Einzel lens comprising intermediate electrode Z, control electrode S and anode A until it reaches the neighborhood of the'lower edge of the intermediate electrode Z. In the long bore of the intermediate electrode Z only the penetration of the electrostatic iield acts; there is, therefore, a long trajectory, in this bore, in a non-homogeneous field. Here arise the severe aberrations menti-oned by Septier. The inferiority of this arrangement is also shown by the fact that images could only be obtained when the potential difference between the control electrode S and the cathode K was at least a tenth of that between they cathode K and the anode A. This initial potential difference, however, is detrimental to the properties tof the Einzel lens, so that good resolution cannot be expected. The long bore in the inter- With these dimensions, it is true, suitableV mediate electrode Z also causes reflections of electrons j from the walls of the bore. These reections make the image formation worse.

These drawbacks are avoided by means of the invention, an embodiment of which is shown in FIG. 4. The immersion objective consists of a cathode K, an intermediate electrode Z, a control electrode S and an anode A. The dimensions in this embodiment are as follows: the distance a is 3 mm. (millimeters); the thickness z of the intermediate electrode is 0.25 mm. (millimeter) in .the middle, but it increases outwardly for greater strength; the diameter of the bore Dz in the intermediate electrode is 0.3 mm. (millimeter). The electrostatic eld between the cathode K and intermediate electrode Z remains homogeneous in this arrangement until near the bore,

and the necessarily non-homogeneous part in and near the bore is kept very short as the beam immediately enters the field of the Einzel lens. The reiiections from the walls of the bore are also reduced.

The characterizing feature of the embodiment described is that the thickness of the intermediate electrode Z is less than a tenth of the distance between cathode K and intermediate electrode Z and the diameter of the bore in theintermediate electrode Z is at most a tenth of the distance between cathode and intermediate electrode.

To reduce the reections from the walls of the bore in the intermediate electrode Z still further, the edges Be of the bore may be rounded as shown in FIG. 6.

Funnel-shaped bores Bf (FIG. 7) are particularly suitable, the narrow part of the bore preferably facing the cathode K.

By selecting the dimensions of the intermediate electrode Z and cathode K in accordance with the invention, conditions are created in which a homogeneous electrostatic field of great field intensity can be produced -between the cathode K and the intermediate electrode Z without regard to the succeeding electrodes. Because of this the image brightness is increased on the one hand and the prerequisites for obtaining an image with good resolution are fulfilled on the other. This image has to be magnified by the Einzel lens. It is therefore essential for the good resolution to be retained as far as possible For this reason a lens with the least possible aberrations should be chosen. Septiers experiments were with a thick Einzel lens as used for projection lenses low in distortion. These projectionsl lenses are not suitable for use as objective lenses, however, because of their other lens errors. The objective shown in FIG. 4 therefore has a thin Einzel lens of the kindkused as objective lenses in transmission microscopes. In this embodiment the intermediate electrode Z, the control electrode S and the anode A are so constructed that together they have the properties of a good electrostatic objective lens.

The small bore in the intermediate electrode Z normally, after manufacture and in use, shows certain deviationsfrom rotational symmetry which make themselves Visible in the image as astigmatism. Such errors in the image can be compensated by suitable transerse displacement or by inclination of the various electrodes towards each other. It is therefore an advantage if one or more electrodes can be adjusted while in use.`

In the embodiment shown in FlG. 4, the field penetration through the bore'in the intermediate electrode Z is very small. The control electrode S therefore really acts as the middle electrode of an Einzel lens and can be operated at a potential identical to or only little different frorn the cathode potential. A special feature of the preferred embodiment is therefore that the potential difference between the control electrode S and the cathode K is less than a tenth of the potential difference between the cathode K and anode A.

This potential difference is preferably made variable within the limits given; it then iniluences the focal length of the immersion objective and can be used for sharpfocussing the image. In one embodiment, therefore, a variable potential which is small compared with the cathode-anode potential difference is applied to the control electrode S to sharp-focus the image. This'can be accomplished by connecting the moving arm W of potentiometer R to the control electrode S as shown in FG. 5.

- It is well known thatY in any immersion objective the best resolution can only be obtained if the rays of wide aperture are projected onto a screen. This screen B is also shown in FIG. 4. In the preferred embodiment, therefore, an aperture screen follows the anode A.

A lower limit is set on the dimensions of the bore in the intermediate electrode Z by the fact that this bore restricts the image size. If the minimum final magnification is set at ldtx and the image diameter at this magnification is set at 60 mm. (millimeters), an approximate lower limit for the bore diameter is rfield, the negative ions which are otherwise masked as to intensity by the electrons stay behind in the beam. The immersion objective is then also suitable for taking ionoptical surface pictures. If all potentials applied to the immersion objective are subjected to polarity reversal, i.e. the cathode and anode interchange, the objective is suitable for image formation with positive ions. The immersion objective of the invention may therefore be adapted to form images both with electrons and with ions.

What I claim is:

l. An electrostatic immersion objective comprising a cathode, an intermediate electrode, a control electrode, and an anode, said cathode, intermediate electrode, control electrode and anode beingserially arranged in spaced Y relation in the aforesaid order, the intermediate and conr trol electrodes and anode having aligned bores, said in the bore of the intermediate electrode, said diameter of the bore of the intermediate electrode being less than one-fth of said distance between the flat surfaces of the. cathode and the intermediate electrode while said thick-- 5 ness of the intermediate electrode at said bore is less than one-fifth ofthe aforesaid distance.

2. The objective as claimed in claim l, in which the thickness of said intermediate electrode is, at least in the region of its bore, a tent'n of the distance between said cathode and said intermediate electrode, and the diameter of the bore in said intermediate electrode is substantially a tenth of the distance between said cathode yand said intermediate electrode.

3. The objective as claimed .in claim in which the edges of the bore of said intermediate electrode are rounded.

4. An objective according to claim l wherein said bore in the intermediate electrode has a funnel shape with a narrow portion facing the cathode.

5. The objective as claimed in claim 1 including means for maintaining a given potential dierence between said anode and said cathode, means for maintaining said intermediate electrode .at substantially the same potential as said anode, and means for maintaining said control electrode at substantially the same potential as said cathode.

6. The objective `as claimed in claim 5 wherein the potential of said anode is greater than the potential of said cathode.

7. The obiective as claimed in claim 1 including means for maintaining a given potential difference between said anode .and said cathode, and means for applying a potential difference between said control electrode `and said cathode having 4a magnitude which is less than `a tenth of said given potential difference between said cathode and said anode.

3. The objective as claimed in claim l including an aperture screen disposed beyond said anode.

UNITED STATES PATENTS References Cited bythe Examiner 2,410,658 11/46 Hillier 250-495 2,536,878 1/51 Fleming Z50- 49.5 2,759,117 8/56 Hasbrouck 250--495 2,814,729 11/ 57 Newberry et al Z50-49 .5

0 RALPH G. NlLSON, Primary Examiner. 

1. AN ELECTROSTATIC IMMERSION OBJECTIVE COMPRISING A CATHODE, AN INTERMEDIATE ELECTRODE, A CONTROL ELECTRODE, AND AN ANODE, SAID CATHODE, INTERMEDIATE ELECTRODE, CONTROL ELECTRODE AND ANODE BEING SERIALLY ARRANGED IN SPACED RELATION IN THE AFORESAID ORDER, THEINTERMEIDATE AND CONTROL ELECTRODES AND ANODE HAVING ALIGNED BORES, DAID CATHODE AND INTERMEDIATE ELECTRODE HAVING FLAT PARALLEL SURFACES WHICH FACE ONE ANOTHER AND ARE SPACED APART A DETERMINABLE DISTANCE, SAID INTERMEDIATE ELECTRODE HAVING A THICKNESS AT SAID BORE AND A DIAMETER OF SAID BORE RELATED TO THE DISTANCE BETWEEN THE FLAT SURFACES OF THE CATHODE AND THE INTERMEDIATE ELECTRODE TO PROVIDE A HOMOGENEOUS FIELD IN THE BORE OF THE INTERMEDIATE ELECTRODE, SAID DIAMETER OF THE BORE OF THE INTERMEDIATE ELECTODE BEING LESS THAN ONE-FIFTH OF SAID DISTANCE BETWEEN THE FLAT SURFACES OF THE CATHODE AND THE INTERMEDIATE ELECTRODE WHILE SAID THICKNESS OF THE INTERMEDIATE ELECTRODE AT SAID BORE IS LESS THAN ONE-FIFTH OF THE AFORESAID DISTANCE. 